Some metallurgical processes involve the treatment of aqueous slurries containing solid, metal-containing particles. In one example of a metallurgical process, an aqueous slurry of a mineral or metal-containing compound is subjected to leaching or oxidation at elevated temperatures and pressures in an autoclave. In order to maximize the efficiency of the leaching or oxidation process, it is preferred to preheat the slurry before feeding it to the autoclave. For this purpose, a preheating vessel is provided upstream of the autoclave. Inside the preheating vessel the slurry is preheated to within a narrow temperature range by contact with a heating medium such as steam generated by an autoclave flashing process.
Typically the slurry is preheated by controlling the flash steam pressure in the apparatus, i.e. by venting, but precise control of the slurry temperature is difficult to achieve in this manner. An improved method and apparatus for preheating slurry is described in U.S. Pat. No. 6,945,775, issued on Sep. 20, 2005 to Fraser et al., in which the temperature of the slurry in the preheating vessel is accurately controlled by heating a first portion of the slurry in the preheating vessel and adding to it a second portion of slurry at lower temperature.
The temperature of the slurry is only one factor influencing the operating temperature within the autoclave. Even with accurately controlled preheating, the slurry discharged from the preheating vessel may be either too hot or too cool for optimal and efficient operation of the metallurgical process inside the autoclave. Under these conditions, it may be necessary to provide auxiliary heating or cooling to ensure that the desired process conditions are met.